1. Field of the Invention
The present invention relates to an image forming apparatus that includes a plurality of a plurality of image bearing members brought into contact with an intermediate transfer unit, such as an intermediate transfer belt.
2. Description of the Related Art
Conventionally, in an image forming apparatus of the above-mentioned kind, a plurality of image forming stations for forming toner images of different colors are arranged in the direction of motion of an intermediate transfer unit, such as an intermediate transfer belt. In the image forming stations, there are arranged image bearing members, such as photosensitive drums, which are driven at predetermined rotational speeds, respectively. Around each image bearing member, there are arranged an electrostatic charger, an image writing section, and a developing device.
While all the image bearing members are being driven for rotation, toner images formed on the respective image bearing members are superposed one upon another on a transfer medium. Then, after the toner images on the transfer medium are transferred onto a conveyed sheet, the toner image are subjected to a fixing process, whereby a color image is formed on the sheet.
In the above-described image forming apparatus, toner images formed on the image bearing members are superposed one upon another on the transfer medium to thereby form a color image, so that unless the toner images are accurately superposed one upon another without color misregistration caused by displacement of the toner images, it is impossible to obtain high-quality color images.
However, it is known that if the center of rotation of a drive source of an image bearing member, such as a photosensitive drum, or the center of rotation of a drive source of the intermediate transfer unit, such as the intermediate transfer belt, is eccentric, the position of a toner image is displaced to cause periodic color misregistration.
To overcome the problem, there has been proposed an image forming apparatus configured to perform control so as to make the respective rotational phases of photosensitive drums in phase (adjust the rotational phase relationship), to thereby prevent toner images from being displaced to suppress periodic color misregistration (see Japanese Patent Laid-Open Publication No. 2006-201255).
In the above-described Japanese Patent Laid-Open Publication No. 2006-201255, however, the control for making the respective rotational phases of the photosensitive drums in phase is performed in a state in which the photosensitive drums and the intermediate transfer belt are in contact with each other. Therefore, insofar as the respective rotational phases of the photosensitive drums are not in phase at the start of the rotational phase control, a speed difference is inevitably caused between the photosensitive drums and the intermediate transfer belt. As a consequence, there occurs slips between the photosensitive drums and the intermediate transfer belt, which damages the photosensitive drums and the intermediate transfer belt.
Further, when a slip occur between each photosensitive drum and the intermediate transfer belt, a resisting force corresponding to the amount of slip acts on the photosensitive drum, and a resisting force corresponding to all the amounts of slips of the photosensitive drums acts on the intermediate transfer belt. This causes large loads to act on the drive sources of the photosensitive drums and the intermediate transfer belt.